1. Field
Embodiments of the invention relate to cancellation of individual logical volumes in premigration chains.
2. Description of the Related Art
In a virtual tape storage system, a host system coupled to cache (e.g., in the form of hard disk storage) and an automated tape library is used to emulate tape drives and tape cartridges (also referred to as “tapes”). In this way, the host system performing Input/Output (I/O) operations to tape is in fact performing I/O operations to the cache emulating the tape storage. Logical volumes in cache are written to physical volumes on the tape cartridges. Typically, multiple logical volumes are stored on each physical volume.
A Virtual Tape Server (VTS) receives and processes I/O operations from the host systems. Premigration may be described as moving data from the cache to a tape cartridge to make room in the cache.
A typical logical volume is written to disk virtual volume cache 180 and “closed” or “rewound and unloaded”. At that time, premigration is scheduled (queued) for the logical volume. At some later, appropriate time, the queued premigration requests are processed (e.g., it may be hours later) as part of a chain. At some later time (e.g., hours or days), the logical volume is removed from the disk virtual volume cache 180 to free space.
Currently, a premigration command specifies a chain of individual logical volumes that are to be premigrated to provide efficient data transfer. For example, the chain may include logical volumes “A, B, C, D, E, F”. With chaining, the logical volumes “A, B, C, D, E, F” all go onto the same tape sequentially and there is no need for physical tape load/unload during the chain. It may take several minutes to transfer data in the chain to the tape cartridge. During this period of time, one of the volumes that was placed on the chain may have to have its transfer cancelled. In such a case, the premigration of the chain is stopped (i.e., any logical volumes in the chain that have not already been transferred are not transferred). For example, if premigration of logical volume “D” is to be cancelled before logical volume “D” has been premigrated, then any logical volumes that have already been transferred to tape cartridge have been successfully premigrated (e.g., logical volumes “A, B, C”), and any logical volumes that have not already been transferred to tape cartridge (e.g., “E, F”) are not transferred (i.e., their premigration fails). In this scenario it may be said that the chain “breaks” at logical volume “D”.
Since logical volumes “E, F” should be premigrated, a separate premigration command is issued for these logical volumes.
Thus, there is a need in the art for improved processing of premigration chains.